We hypothesize that amyloid B-protein (ABeta) assembly into oligomers and polymers is a seminal neuropathogenetic process in Alzheimer's disease (AD) and in cerebral amyloid angiopathy (CAA). Inhibiting formation of, or disrupting, ABeta assemblies thus could be of benefit in the treatment of these disorders. To test this hypothesis, detailed knowledge of the folding and assembly of ABeta is necessary. In this proposal, we seek to understand the structural and kinetic features of ABeta fibrillogenesis in order to facilitate later rational design and testing of therapeutic agents. Our prior work in this area has contributed to the discovery and characterization of heretofore-unrecognized conformational and structural intermediates in the assembly process, e.g., protofibrils. In fact, protofibrils have been found to be toxic to cultured neurons and new studies suggest that protofibrils may be key pathogenetic effectors of the Arctic form of familial Alzheimer's disease. Here, we have proposed an ambitious plan that seeks to eventually provide a rigorous structural and thermodynamic elucidation of the entire pathway through which nascent ABeta folds and assembles. Key areas of focus are the early oligomerization of ABeta monomer, the formation of an a-helix-containing conformational intermediate, protofibril assembly, assembly-dependent changes in the topology of amino acids in ABeta, and how the biophysical effects of AD- and CAA-linked amino acid substitutions correlate with the disease phenotype. These studies will contribute to our understanding of AD and CAA. In addition, because amyloid assemblies from most, if not all, of the -20 different kinds of amyloidoses share certain common structural features, the data generated here also should be relevant to these other disorders. Three Specific Aims are proposed: Aim 1. To characterize the conformational, morphologic, and assembly dynamics associated with key intermediates in fibril formation. Aim 2. To determine the topological organization of amino acids during ABeta folding and assembly. Aim 3. To determine the effects of novel AD- and CAA-associated amino acid substitutions on ABeta folding and assembly.